Biogas is a biofuel produced by anaerobic digestion that has been receiving increased attention in recent years. Biogas contains predominantly biomethane and is commonly used as heating fuel or for electricity generation, although it can potentially also be used as a transportation fuel or as an intermediate to produce another transportation fuel. A benefit of making biogas is that a wide variety of feedstocks can be used to produce the gas, including landfill waste or waste streams from commercial plants. In the production of ethanol from cellulosic feedstocks, waste streams remaining after the recovery of ethanol are often treated by anaerobic digestion to produce biogas. Landfills also produce biogas through anaerobic digestion of municipal waste. While the biogas is commonly used on-site for heating or electricity generation, for example at a plant or a landfill site, its use is less widespread in the transportation sector. However, commercializing the use of biogas in other applications besides on-site use at a plant or other facility would be desirable.
There are several practical barriers to using biogas off-site as a transportation fuel. For instance, the gas generally needs to be transported from the location where it is made to a location where it can be used as a transportation fuel, which is often remote from the biogas source. The most common and typically least expensive way to transfer methane such long distances is by natural gas pipeline. There are large networks of natural gas pipeline that already exist and using these pipelines saves significant expense associated with building an alternative pipeline dedicated for transporting biogas.
However, prior to introducing a gas comprising biomethane to such pipelines, it is typically required to be purified to pipeline quality specifications. Furthermore, for its use in transportation applications, biomethane must be purified sufficiently so that it is compatible with vehicle engines. The purity of natural gas is often quantified as a heating value in British Thermal Units (BTU)/cubic foot. The higher the heating value, the cleaner the natural gas due to a reduced concentration of non-combustible gases. To introduce natural gas into a pipeline, including from fossil sources, it is typical that pipeline operators require clean-up to meet their specifications, including requiring the input gas to be above a heating value of, for example, 950 BTU/cubic foot. Biogas in its crude form has a low heating value compared to pipeline quality natural gas and thus requires significant purification.
While methods exist for purification of biogas to meet pipeline specifications, or to enable direct use in transportation applications, they are expensive to operate and require significant capital investment. Impurities in biogas include carbon dioxide, hydrogen sulfide, water, oxygen, siloxanes, nitrogen and/or halogenated compounds, depending on its source. Impurity removal from the crude biogas may be carried out by scrubbing techniques such as water or polyethylene glycol scrubbing, which involve flowing biogas through a column with a water or polyethylene glycol solution flowing counter-current to the biogas. A further technique for impurity removal, such as carbon dioxide removal, from the crude biogas is pressure swing absorption, which utilizes adsorptive materials, such as zeolites and activated carbon that preferentially adsorb carbon dioxide at high pressure. When the pressure is released, the carbon dioxide desorbs. Membrane separation is another technique that can be used to remove impurities from the crude biogas. Membrane separation may include high pressure gas separation or gas-liquid absorption membranes. However, each of these techniques requires the use of specialized equipment and/or solvents.
Despite the challenges with commercializing biogas for transportation use, an advantage of using it in such applications is that it enables the generation of valuable fuel credits, which otherwise may not be available if biogas were simply used at a facility for heat or electricity. The utilization of renewable fuels for transportation applications has been promoted by various governments, including the United States government through the Energy Independence and Security Act (“EISA”) of 2007. Some of the purposes of the act are to increase the production of clean renewable fuels, to promote research on and deploy greenhouse gas (“GHG”) capture and to reduce fossil fuels present in fuels. Valuable fuel credits can be available when transportation fuels have a reduced GHG footprint. However, since fuel credits are generated for biogas in association with its use for transportation, in order to obtain such credits, one must typically incur the capital expense associated with purifying the gas to at least 950 BTU/cubic foot.
Thus, there is a need in the art for a process that can reduce the cost of using methane sourced from biogas for transportation fuel, which in turn can enable the generation of valuable fuel credits with reduced capital cost. Further, there is a need in the art to incentivize the utilization of biogas so as to reduce GHG emissions. The more widespread the use of biogas, the less natural gas methane is utilized, which in turn contributes to lower GHG emissions.